A 5-mm diameter spherical ball at 60'C is covered by a 1-mm thick plastic insulation (k=0.13 W/mC). The ball is exposed to a medium at 10'C, with a combined convection and radiation heat transfer coefficient of 20W/m^2C. Plot the thickness of the plastic insulation as a function of the heat transfer up to and slightly beyond where it helps then hurts the heat transfer from the ball.